A current vehicle skeleton member generally has a structure in which steel is subjected to sheet metal processing and assembled by spot welding.
Although the vehicle skeleton member has a very complex shape for providing a reinforcing structure for increasing body rigidity and a living space, a large limitation is not needed on the shape of a joint part owing to a robotized spot-welding technology, and therefore a high productivity is enabled.
On the other hand, in recent years, for decreasing environmental burdens of automobiles, saving of body weight has been strongly desired and application of a carbon fiber composite material to vehicle skeleton members has been attempted. In these vehicle skeleton members made from the carbon fiber composite material, their weight has been drastically saved as compared with the case of existing steel-made vehicle skeleton members. However, since the shapes of the members have to follow complex shape of the existing steel-made vehicle skeleton members, an expensive carbon fiber woven fabric is to be used and shaping thereof is performed by a hand lay-up or autoclave method that is low in productivity. Therefore, from the viewpoint of economic efficiency, wide range of use thereof has been extremely limited. Recently, although an improvement in productivity is attempted using RTM method (Resin Transfer Molding method) or the like (see, Patent Document 1), but it is hard to say that the technology can be applied to general vehicles, since a curing reaction time of a thermosetting resin used as a matrix is rate-limiting.
Moreover, at the application of the fiber-reinforced composite material to the vehicle skeleton member, the joint of the fiber-reinforced composite materials themselves and the joint of the fiber-reinforced composite material to the other material such as a metal become a problem. Although the fiber-reinforced composite material is light in weight and has a high strength, it is not preferable to apply a local load such as volt fastening, so that there have been proposed a large number of structures for dispersing the load at a fastening portion. Patent Document 2 shows examples of structures for attaching the fiber-reinforced composite material. The stress concentration at the joint portion can be avoided by these structures, but an improvement in productivity is limited since the structures are complex.
On the other hand, as a means for improving the productivity of fiber-reinforced composite materials, a thermoplastic fiber-reinforced composite material containing a thermoplastic resin used as a matrix has been developed. Such a thermoplastic fiber-reinforced composite material enables shaping within a short tact time by stamping after heated and plasticized. Since pressing pressure required for stamping is lower than that required for stamping molding of steel, integral molding is applicable in the case of such a size as a floor for a vehicle. Moreover, since the thermoplastic fiber-reinforced composite material can be softened by re-heating, it is also one characteristic that jointing by welding is possible.    (Patent Document 1) JP-A-2008-68720    (Patent Document 2) JP-A-2006-64010